Comma Setting Off Restrictive Clauses

A restrictive clause is a clause which limits a subject; this might be something identifying, such as the colour of a person’s hair, or the position of a book on a table. A restrictive clause may use the word that, and cannot be removed from the sentence without loss of information.

The box of apples, that has the red label on it, can be used to make the pie.

The restrictive clause that has the red label on it should not have a comma on either side of it. The label is required to identify the box.